The present invention relates to a magnetic recording medium having a magnetic layer on one surface of a non-magnetic supporting substrate and a back coat layer on the other surface thereof, in which non-magnetic powders are dispersed in a binder.
So far, magnetic recording media of the so-called coated type have been widely used as magnetic recording media. A coated type magnetic recording medium is typically fabricated by coating a supporting substrate with a magnetic coating material in which ferromagnetic oxide powders of .gamma.-Fe.sub.2 O.sub.3 or .gamma.-Fe.sub.2 O.sub.3 containing cobalt, or a magnetic powder material such as alloy magnetic powders composed mainly of iron, cobalt and nickel is dispersed in an organic binder such as a vinyl chloride-vinyl acetate copolymer, polyester resin, and polyurethane resin, followed by drying.
To meet steadily increasing demands for ever-higher recording density and size reductions, magnetic recording media are now required to have ever-higher smooth and ever-thinner structures. In the coated type media, especially in media using alloy magnetic powders, therefore, making magnetic layers very smooth and thin is under investigation.
Apart from this, a magnetic recording medium of the so-called ferromagnetic metal thin film type has been proposed, in which a cobalt, iron, or Co--Ni alloy base ferromagnetic metal material is directly deposited on a nonmagnetic supporting substrate by vacuum thin film technologies such as vacuum evaporation and a lubricating agent layer is provided on the ferromagnetic metal material. This magnetic recording medium is mainly put to practical use as a magnetic recording medium for commercial video cameras.
This ferromagnetic metal thin film type of magnetic recording medium is a very advantageous medium in view of electromagnetic characteristics because the packing density of the magnetic material can be increased for reason of no need of incorporating a non-magnetic organic binder into the magnetic layer, and the thickness of the magnetic layer can be much more reduced, so that demagnetization upon recording and thickness loss can be much more reduced than ever before.
However, the recording medium having a ferromagnetic metal thin film merely deposited on a nonmagnetic supporting substrate together with a lubricating agent layer has problems regarding reliability such as still durability, and the weather resistance of a magnetic layer.
To provide a drastic solution to these problems, the use of a protective layer is under investigation. In particular, a magnetic recording medium provided with a hard carbon film by plasma polymerization has been put forward, and is row practically used as a magnetic recording medium for commercial digital VTRs.
To make sure of the running performance of these high-density recording media in decks, they are generally provided with a back coat layer irrespective of whether they are of the coated type or the metal thin film type, and irrespective of whether or not the metal thin film media are provided with a protective film.
The back coat layer is first required to make sure of the running performance of the recording medium in a deck. For data storage, however, special care must be taken to maintain the initial performance even after long-term storage or storage at high temperature and humidity. During storage, the back coat layer comes into direct contact with the magnetic layer on the other surface of the supporting substrate or is in opposition thereto though the protective film or lubricating layer. Therefore, when the back coat layer has too coarse a surface shape, the surface shape is transferred onto the magnetic layer, not only resulting in an increased drop-out but making error rates worse as well. In some cases or although depending on the material of which the back coat layer is formed, the back coat layer may stick to the magnetic layer or may launch a corrosive attack on the magnetic layer. Referring here to a magnetic layer of a high-density recording medium in particular, there is now no choice but to mirror-finish the magnetic layer, and the magnetic layer is often designed such as to match the coated type medium or the metal thin film type, both using alloy magnetic powders. To meet the aforesaid storability, therefore, it is required to satisfy ever-severer demands for the back coat.
In general, such a back coat layer has a structure wherein non-magnetic powders are dispersed in a binder. In many cases, especially carbon black is dispersed as a main component in the binder optionally with the addition thereto of other carbon black species or various pigments or various additives. The reasons for using carbon blacks for the back coat layer are to lower the surface electric resistance of the back coat layer thereby preventing deposition of dust by static electricity, impart a light-blocking property to the back coat layer thereby preventing malfunction, and improve running durability.
One example of using carbon black as non-magnetic powders is disclosed in JP-B 52-17401. The objectives of this example are to make use of the conductivity of carbon black thereby achieving antistatic and light-blocking effects, and make use of the agglomeration of carbon particles thereby making the surface of a back coat layer rough. However, this carbon black can hardly be prepared in a coating material form and so is prone to agglomeration because its average particle diameter ranges from 10 to 20 nm. An agglomeration of carbon black particles, in turn, gives asperities on the magnetic layer. Since carbon black having a small particle diameter is poor in dispersibility, it is difficult to allow the carbon black to have such an average roughness as to prevent the formation of the aforesaid asperities. The reason why it is difficult to disperse carbon black having too small an average particle diameter appears to be that such carbon black tends to have a secondary structure.
JP-A 63-144416 shows an example of using carbon black having a large particle diameter. The carbon black used therein is of the thermal type having a particle diameter of 60 to 200 nm. This structure-free type carbon black can be uniformly dispersed in a binder and has some striking effect on lowering the coefficient of friction. However, this carbon black is not suitable for a back coat of a high-density recording medium because the size of carbon black particles upon uniform dispersion still imposes limitations on reducing the formation of asperities.
JP-B 2-49490 shows an example of using a plurality of carbon black species. A main objective of this example is to improve wear resistance. Carbon black having a particle diameter of 10 to 35 nm is used in combination with carbon black having a particle diameter of 40 to 150 nm. JP-B 4-81261 makes investigations on the combined use of fine carbon black having an average particle size of 30 to 100 nm and coarse carbon black having an average particle size of 150 to 500 nm. Even when carbon black species are used together while only the particle diameter difference is taken into account, however, it is difficult to arrive at a reasonable compromise between the running performance and storability of a high-density recording medium.
As one specific example, JP-B 2-49490 refers to the combined use of 70% by weight of carbon black having a particle diameter of 26 nm and 30% by weight of carbon black having a particle diameter of 64 nm. In this example, however, there is a large particle diameter difference between both the carbon black species. Thus, the carbon black having a larger particle diameter appears as a multiplicity of soike protuberances on the back coat surface formed by the carbon black having a smaller particle diameter, offering a problem regarding transfer of the back coat surface onto the associated magnetic layer. On the other hand, JP-B 4-81251 refers as one specific example to the combined use of fine carbon black -having an average particle size of 50 nm and coarse carbon black having an average particle size of 150 nm. However, the particle diameter of the coarse carbon black is too large and so there is a large particle diameter difference between the fine particles and the coarse particles. In this case, too, a problem arises regarding transfer of the back coat surface onto the associated magnetic layer due to the formation of spiky protuberances.
In many attempts made so far in the art, carbon black is used as non-magnetic powders along with other inorganic pigment. For instance, JP-B 5-72647 makes investigations on the addition of carbon black and an inorganic pigment having a Mohs hardness of up to 6, e.g., CaCO.sub.3, BaSO.sub.4 or Fe.sub.2 O.sub.3 for the purpose of reducing a wear on the back coat surface. Since a vinyl chloride copolymer or cellulosic resin is used as the binder, however, it is not preferable to use this non-magnetic powder system as a back coat of a coated type medium or a metal thin film type medium, both using alloy magnetic powders, because a problem arises upon storage at high temperature and humidity.
In some attempts so far made in the art, species of carbon are used in combination with other inorganic pigments. JP-A's 2-42624, 2-42625, 2-134720 and 2-141925 disclose the combined use of carbon black having an average primary particle diameter of 20 to 40 nm, carbon black having an average primary particle diameter of 50 to 100 nm, and other inorganic pigment. In all these attempts, however, cellulosic resins are used as the binders. It is again not preferable to use such systems as a back coat of the coated type medium or the metal thin film type medium, both using alloy magnetic powders, because a problem arises upon storage at high temperature and humidity. This inorganic pigment is provided to give a moderate surface roughness to the back coat surface, and so has an average particle diameter of 300 to 1,500 nm that poses a problem regarding transfer of the back coat surface onto the associated magnetic layer.
Binders-excellent in terms of the dispersibility of non-magnetic powders therein, adhesion thereof to non-magnetic supporting substrates, and the wear resistance of coated films are now used. For instance, one or at least two resins of thermoplastic resins such as polyurethane resin, polyester resin, cellulosic resin, vinyl chloride copolymer resin and phenoxy resin are used in combination with a polyisocyanate compound to obtain a heat curable type binder, and in combination with a resin having an unsaturated double bond sensitive to radiation to obtain a radiation curable type binder.
However, the vinyl chloride copolymer resin and cellulosic resin, both tending to incur the risk of giving off corrosive gases, are not preferable for a binder for the back coat of the coated type medium or metal thin film type medium, both using alloy magnetic powders.
A binder containing neither a vinyl chloride copolymer resin nor a cellulosic resin is disclosed in JP-A 58-200426 as well as in JP-A 59-2228. The binder is made up of phenoxy resin, thermoplastic polyurethane elastomer and polyisocyanate. However, this binder system has a primary effect on the initial running durability, and the publications say nothing about whether or not it has an effect on storability. The publications make no particular reference to the dispersibility of the polyurethane elastomer used, and the polyurethane elastomer exemplified has no polar group. A binder comprising a combination of this type of polyurethane elastomer with phenoxy resin is found to be insufficient in terms of the dispersion of inorganic powders. The filler used is an inorganic powder such as CaCO.sub.3 powder or carbon black. The publications teach the sole use of these materials, but fail to give any suggestion as to using a mixture of two or more materials. It is thus difficult to arrive at a sensible tradeoff between the prevention of deposition of dust by static electricity that is achieved by making sure of conductivity thereby lowering the surface electric resistance and the wear resistance of the back coat surface. In this connection, it is noted that while many proposals have been made as to techniques for introducing polar groups in the polyurethane elastomer, nothing is still proposed to improve the dispersibility of carbon black in a mixture system of polyurethane elastomer and phenoxy resin.
It is noted that JP-B 1-91317 and JP-A' 6-325353, 7-169040 and 8-17037 disclose an amine-containing polyurethane. However, these publications fail to disclose the particle diameter of carbon black, and provide no disclosure about the combined use of carbon black and phenoxy resin or inorganic pigment.